Question: Let $X$ and $Y$ be sets: $X = \{15,9,11\}$ $Y = \{9,11\}$ What is the set $X \setminus Y$ ?
Answer: Remember that $\backslash$ refers to the difference between sets. The difference of two sets $X$ and $Y$ is the collection of elements which are in $X$ but not in $Y$ The members of a set must be unique, and the order doesn't matter. $X \setminus Y = \{15\}$